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Full text of "Handbook Of Chemical Engineering - I"

122
CHEMICAL ENGINEERING
veniently and thus the point of highest economy determined. The starting pressure = Sa ft., where Sa = submergence when not pumping, is generally higher than the operating pressure. In Fig. 15, a = Pohle or side inlet, b = annular space inlet, c — Saunders system, d = central air-pipe system.
The efficiency of the air lift is very low, but its many advantages, viz. simplicity, absence of working parts in the well, ability to handle gritty water or oil, and principally the fact that the full area of the well may be rendered available tend to counteract its low duty. Generally it is desirable to work a well to the limit of its capacity, regardless of expense.
TABLE 4.—AIR-LIFT DATA*
Lift, feet, I	Water horsepower, W	Displacement of air compressor, cubic feet per minute, D	Operating air pressure, pounds per square inch	Air, horsepower, adiabatic, A	Indicated steam horsepower, /	Over-all efficiency, per cent, 100 F
20	0.00505	0.38	13	0.017	0.0213	23.7
40	0.0101	0.475	26	0.036	0.045	22.5
80	0.0202	0.68	52	0.0825	0.103	19.6
120	0.0303	0.82	78	0.129	0.162	18.8
160	0.0404	0.97	104	0.178	0.223	18.2
200	0.0505	1.13	130	0.235	0.294	17.2
250	0.063	1.44	162	0.335	0.418	15.0
The best efficiency is obtained for a capacity of from 10 to 15 gal. per minute per square inch of drop pipe (allowing for an obstruction of 20 per cent by the central air pipe, this will be equivalent to a water velocity of from 3.2 to 4.8 ft. per second.
Another method of air pumping is to admit compressed air to the steam end of a steam pump. This can be done without difficulty in a simple cylinder direct-acting pump, but where the air is used expansively, the moisture in the air is apt to freeze at the exhaust nozzle and choke the exit. A remedy is to heat the air before admission, and this also effects a considerable increase in efficiency which at best is quite low.
The hydraulic ram is used for lifting small quantities of water against high heads, where an abundance of drive water is available. The usual capacities are from 1 to 30 gal. per minute. The drive water flows through a pipe and out through a clack valve. As soon as a certain velocity is attained the valve closes and the water continues to flow by inertia and rises to a higher level in the discharge pipe. The operation continues automatically. In practice it is impossible to lift water by a ram to a height over 20 times the head H. The drive pipe should be unobstructed and have a length in feet = (H + h + 2)2h + //, and not less than = 5H. An average velocity of 0.8 ft. per second should be secured in the drive pipe. The volume of the air chamber should be approximately equal to that of the discharge pipe. There are two efficiency formulas used for rams, one by Rankine, giving the commercial efficiency E = qh ~ QH,
1 Based on a submergence of 1.51, this being considered the most desirable submergence. Figures in table are for a capacity of 1 gal. per min.